Any reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge in Australia or elsewhere.
Ultraviolet light (UV) and visible light absorbing or screening compounds have found use in a range of applications where protection from the sun's harmful UV rays is desirable. This includes their use in glass and lens coatings, paints, packaging, household cleaning formulations and materials including fabrics as well as, perhaps most notably, in sun screen formulations to protect the skin of the user from damage caused by UV radiation.
Compounds suitable for absorbing UV light, and therefore offering a protective function, were described in WO 2015/006803 in the name of the present applicant. The compounds disclosed therein displayed a cyclic enaminoketone core with a range of substitutions designed to provide a useful variance in absorption characteristics to provide for coverage throughout the UV range. While effective for their primary purpose the stability of the compounds to exposure to electromagnetic radiation was not considered or discussed in any form.
Inorganic sunscreens, such as zinc oxide, protect from UV primarily by light scattering. Conversely organic sunscreen agents, such as those described in WO 2015/006803, absorb the UV light and must then emit this energy in order to return to the ground state. Such energy loss can occur by any or all of a variety of pathways such as fluorescence, phosphorescence, transfer to another molecule, isomerisation, heat generation or fragmentation. If destructive pathways such as fragmentation, and some isomerisations, predominate then the ability of the molecule to continue to absorb UV or visible light is destroyed. While protective compounds are not required to be indefinitely photostable it is important that they provide for a useful lifetime and so resist fast degradation.
It would therefore be desirable to provide for compounds which can absorb energy from a variety of ranges within the electromagnetic spectrum and which can demonstrate improved stability to this exposure to provide for a greater operational lifespan.
Further, there is an ongoing need for compounds, whether preferentially stabilised or not, which provide for effective absorption of electromagnetic energy in different regions of the spectrum, particularly in the UV and visible light parts of the spectrum.